[PD] Re: hanging hid object?

[Hans-Christoph Steiner]

We should keep this on the list since other people besides me can  
answer this question, perhaps quicker too, and it might be of general  
interest.

On Jul 6, 2006, at 1:55 PM, pix wrote:

> hey hans,
>
> i was wondering if you have experienced this behaviour where, on  
> linux,
> the hid the hid object hangs if the device disappears (even only
> briefly) while hid has it open?
>
> i thought it might have just been the problem that the device was
> getting a new device file name when it returned, but i have now done
> some udev trickery to make sure the steim junxion box we are using
> always gets the same event number, and the problem still persists.
>
> once the device disappears hid no longer responds to close or open
> messages.
>
> it would be cool if there was a work around because we occasionally  
> have
> problems with the junxion box we are using 'winking out' for just long
> enough for linux to kill off the device.

Hmm, I haven't seen that, but its not surprising that it would hang.   
Basically, the object is reading that file handle every 5 ms, or  
whatever you set the poll time to.  So if sometime reads from a non- 
existant filehandle that's open, then it'll hang, no matter how you  
are doing it.

I have used the junxionbox some and I haven't seen that problem.  I  
think the best way to solve that problem would be to figure out  
what's causing the junxionbox to wink out.  Its in beta test so I  
think they would be interested to hear about that problem.  I was  
just at STEIM for three weeks working with Rene and Taku, who are  
working on the junxionbox.  Taku performs with his junxion-related  
devices, so he's dealt with a lot of these issues.

I can't think of anything offhand as an easy workaround.  You might  
be able to test the filehandle before reading from it, but I am  
worried about the overhead that might introduce.  A key part of hid  
is making it as low latency as possible, and not to take a ton of  
CPU.  Such a check might be pretty minimal but running it 200 times a  
second adds up.  Can any of the big time Linux heads think of a way  
to deal with this?

.hc

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